Exhaust gas from an internal combustion engine, such as an automobile engine, contains nitrogen oxide (NOx), carbon monoxide (CO), hydrocarbon (HC) and the like. These substances can be purified by an exhaust gas purifying catalyst capable of oxidizing CO and HC and at the same time, reducing NOx. As a representative exhaust gas purifying catalyst, it is known to use a three-way catalyst comprising a porous metal oxide support, such as γ-alumina and a noble metal, such as platinum (Pt), rhodium (Rh) and palladium (Pd) supported thereon.
However, such a noble metal is expensive, and may be in short supply in the future due to a reduction in reserves. Accordingly, it is proposed to use, as a catalyst for purifying an exhaust gas, a perovskite-type composite metal oxide, which is a catalyst other than a noble metal.
In this regard, for example, Japanese Unexamined Patent Publication No. 8-229404 discloses using a mixture of a catalyst component containing a noble metal and a catalyst component which is a perovskite-type composite metal oxide having a chemical formula ABO3 (wherein A is selected from the group consisting of lanthanum La, strontium Sr, cerium Ce, barium Ba, calcium Ca and a combination thereof; B is selected from the group consisting of cobalt Co, iron Fe, nickel Ni, chromium Cr, manganese Mn, magnesium Mg and a combination thereof; and O is oxygen). Japanese Unexamined Patent Publication No. 8-229404 describes that the perovskite-type composite metal oxide allows a particulate carbon substance and/or a hydrocarbon to burn from a low temperature region, and promotes oxidation from NO to NO2, which is then temporarily absorbed as NOx.
Japanese Unexamined Patent Publication No. 8-229404 above teaches that the perovskite-type composite metal oxide can be produced by a first step of preparing an aqueous solution comprising dissolved salts of metals constituting the perovskite-type composite metal oxide and a citric acid, a second step of drying the obtained aqueous solution to form a citric acid complex of the metals, a third step of heating/preliminarily firing the citric acid complex at 350° C. or more in a vacuum or inert gas atmosphere, and then a fourth step of firing the heated/preliminarily fired precursor in an oxidative atmosphere.
The porous metal oxide support for loading a catalyst component thereon is not limited only to aluminum, but use of porous silica is also proposed.
For example, Japanese Unexamined Patent Publication No. 2000-24503 proposes an exhaust gas purifying catalyst, wherein the support is a porous silica body having only mesopores of 1 to 5 nm in diameter as the pore, and a noble metal is supported at least in the mesopore. Japanese Unexamined Patent Publication No. 2000-24503 describes that in such an exhaust gas purifying catalyst, the noble metal supported in the mesopore does not readily migrate, and therefore grain growth during an endurance test is suppressed.
Japanese Unexamined Patent Publication No. 2000-24516 proposes an exhaust gas purifying catalyst obtained by loading a noble metal on a support comprising a porous silica body having mesopores of 4 nm or less, wherein a noble metal such as platinum and a metal oxide having an oxygen storing-releasing ability, such as ceria, are supported in the mesopore. Japanese Unexamined Patent Publication No. 2000-24516 states that, in such an exhaust gas purifying catalyst, the noble metal and metal oxide can hardly migrate from the mesopore and are restricted in movement. Therefore, since they are prevented from grain growth into a size larger than the diameter of the mesopore, the highly dispersed state of the noble metal and metal oxide is maintained even after an endurance test.
Incidentally, as the production method of the porous silica, various production methods are known.
For example, Japanese Unexamined Patent Publication No. 2003-181282 above proposes a novel production method of a porous silica used as an adsorbent, the porous silica having a large specific surface area and uniform-size mesopores. In the production method of Japanese Unexamined Patent Publication No. 2003-181282 above, the porous silica is produced by removing alkylamines from an organic-inorganic composite comprising a polycondensate of alkoxysilanes and a micelle-forming alkylamine.
As described in Japanese Unexamined Patent Publication No. 8-229404, in the field of an exhaust gas purifying catalyst, it is known to use a perovskite-type composite metal oxide as the catalyst component. The perovskite-type composite metal oxide used in the field of an exhaust gas purifying catalyst is generally produced by co-precipitating a precursor of the perovskite-type composite metal oxide from a solution containing a salt of a metal constituting the perovskite-type composite metal oxide, and then drying and firing the precursor.
It is known that, with regard to low-temperature catalytic activity, such a perovskite-type composite metal oxide known provides a performance comparable to a noble metal catalyst or better than a noble metal catalyst. However, in terms of the maximum NOx purification percentage, i.e., the NOx purification percentage at high temperatures, good results are necessarily provided.
In the case of obtaining a perovskite-type composite metal oxide by a co-precipitation method as above, the obtained perovskite-type composite metal oxide particle sometimes has a relatively large particle diameter of up to several micrometer (μm). Furthermore, the perovskite-type composite metal oxide sometimes undergoes grain growth during use thereof, resulting in a small surface area.
Accordingly, the present invention provides a perovskite-type composite metal oxide-based exhaust gas purifying catalyst, which has an excellent performance in terms of maximum purification percentage and/or catalytic activity at a relatively high temperature, and is able to prevent the perovskite-type composite metal oxide from undergoing grain growth during use of the exhaust gas purifying catalyst.